Optimized Charge Transport in Molecular Semiconductors by Control of Fluid Dynamics and Crystallization in Meniscus-Guided Coating

Solution processable organic semiconductors (OSCs) hold advantages for the fabrication of flexible and large-area electronic devices. Control over morphology and molecular packing of OSCs is crucial to realize excellent charge carrier transport in organic field-effect transistors (OFET). Meniscus-guided coating (MGC) methods, such as zone casting, dip coating and solution shearing, are scalable laboratory models for large-area solution coating of functional materials for thin-film electronics. Unfortunately, the general lack of understanding of how the coating parameters affect the dry-film morphology upholds trial-and-error experimentation and delays lab-to-fab translation.<br/><br/>In this combined experimental and modelling study, we present a predictive model for structure formation upon crystallization of a (generic) molecular semiconductor during MGC, driven by solvent evaporation. Predicted surface morphologies are fully reproducible by an experimental data which show correlation between coating speed, domain growth, long-range alignment, and charge carrier transport.[1,2] With increasing casting speed, we identified three morphological subregimes; I) an isotropic domain-like structure, II) a band-like structure following the coating direction and III) a corrugated morphology lacking directionality[3].<br/><br/>We interpret our experiments using numerical simulations of the steady state fluid dynamics in the bead and the morphology formation in the deposited film, focusing on the onset of the appearance of unidirectionality. We reveal a direct correlation between the trap density in the OSC film and the casting speed and show how this allows us to achieve an improved saturation and effective charge carrier mobility, with a high reliability factor.<br/>The found correlations are crucial for the development of MGC as a practical processing technique for upscaling the solution deposition of organic semiconductors in future.<br/><br/>[1] J. J. Michels <i>et al.</i>, <i>Nat. Mater.</i>, <b>2021</b>, 20, 68–75<br/>[2] K. Zhang <i>et al. </i><i>Adv. Electron. Mater., </i><b>2021</b><i>, </i>2100397, doi.org/10.1002/aelm.202100397<br/>[3] O. Yildiz <i>et al. Adv. Funct. Mater.</i> <b>2021</b>, 2107976 DOI: 10.1002/adfm.202107976<br/><br/>T.M. acknowledges the Foundation for Polish Science financed by the European Union under the European Regional Development Fund (POIR.04.04.00-00-3ED8/17).